The present invention relates to a gate valve. More particularly, the invention relates to a gate valve which comprises a number of features which contribute to a reduction in the weight and manufacturing cost of the gate valve.
Prior art gate valves, especially those used in the oil and gas industry, typically comprise a valve body, a flow passage extending axially through the body between an inlet port and an outlet port, a gate cavity extending partially through the body generally transverse to the flow passage, and a bonnet mounted to the body over the gate cavity. A valve stem is rotationally supported in the bonnet or in a bonnet cap secured to the top of the bonnet. One end of the valve stem is connected to a handle or other means for rotating the valve stem, and the other end of the valve stem is threaded into a gate which is disposed between a pair of seats that are mounted in the flow passage. Each seat comprises a through bore which is concentric with the flow passage, and the gate includes a transverse opening extending therethrough. In operation, the stem is rotated to raise or lower the gate and bring the opening into or out of alignment with the through bores to either open or close the flow passage, respectively.
In the closed position of such prior art gate valves, the gate will typically seal against the downstream seat, and the upstream pressure will be contained within the valve body by both this gate-to-seat seal and by an annular sealing member disposed between the valve stem and the bonnet. The front face of each seat is usually specially machined or treated to ensure an effective metal-to-metal seal with the gate, and the back face of the seat is often designed to accommodate an additional backup seal with the valve body. Thus, these seats require multiple, time consuming machining steps. Additionally,.these seats are asymmetrical and can only be mounted in the gate valve in a specific orientation.
The valve stem sealing member of prior art gate valves is typically a stem packing which is comprised of many individual sealing elements. Such stem packings are relatively expensive and contribute to the overall complexity of the gate valve. In addition, the stem packing is usually secured within the bonnet by a retainer which is threaded into the bonnet. While this retainer is effective to transmit the upward force acting on the seal to the bonnet, the threaded connection between the retainer and the bonnet is time consuming and costly to manufacture.
In many prior art gate valves, the stem is connected to the gate with an acme thread. Although this is a robust connection, the process of machining the acme threads on the stem and the gate is expensive. Also, in order to permit the gate to float toward the downstream seat when the gate valve is closed, the stem is usually threaded into a lift nut which is loosely retained in the gate. This lift nut necessarily increases the cost and complexity of such gate valves.
In high pressure applications, considerable upward thrust is imparted on the valve stem. Thus, prior art gate valves often comprise one or more metallic thrust bearings, such as needle bearings, connected between the stem and the bonnet cap to transfer the thrust load to the bonnet and thereby lower the operating torque of the gate valve. These metallic thrust bearings are relatively expensive and add to the overall weight of the gate valve. In addition, these metallic thrust bearings periodically require lubrication during the life of the gate valve and are also subject to rusting.
Furthermore, in many prior art gate valves the gate cavity comprises a bore having a uniform cross section in the plane perpendicular to the valve stem. However, this cross sectional area is usually larger than is required to accommodate the gate below the flow passage. Therefore, these prior art gate valves are constructed with bodies which are larger than required, and this needlessly increases the cost and weight of the gate valve.
These and other disadvantages in the prior art are overcome by providing a gate valve comprising a valve body having an internal flow passage which extends between an inlet port and an outlet port, a pair of seats mounted in respective seat pockets formed in the body coaxial with the flow passage, a gate disposed between the seats and having a transverse opening therethrough, and a valve stem threadedly connected to the gate, wherein rotation of the valve stem raises or lowers the gate to bring the opening into or out of alignment with the flow passage to either open or close the gate valve, respectively, and wherein each seat comprises a generally cylindrical body portion having a substantially uniform outer diameter and first and second end faces which are adapted to seal with the gate, whereby each seat may be mounted in its seat pocket with either the first face or the second face adjacent the gate.
In accordance with another embodiment of the invention, the gate valve comprises a gate cavity extending through the valve body and intersecting the flow passage, a bonnet connected to the valve body over the gate cavity, the bonnet having a bore extending therethrough which communicates with the gate cavity, a bonnet cap secured to the bonnet over the bore, the valve stem comprising a first portion which is rotationally supported in the bonnet cap and a second portion which extends through the bore in the bonnet and into the gate cavity, an annular sealing member disposed between the valve stem and the bonnet bore, and a retainer for restricting axial movement of the sealing member within the bore. The retainer comprises a radial flange which is secured between the bonnet and the bonnet cap, such that axial forces exerted on the retainer by the sealing member are transmitted to the bonnet cap via the flange and without the need for a threaded connection between the retainer and the bonnet.
In accordance with another embodiment of the invention, the second portion of the valve stem is provided with xe2x80x9cVxe2x80x9d threads that are adapted to engage corresponding xe2x80x9cVxe2x80x9d threads formed in a longitudinal hole extending into the gate from a top surface thereof. The xe2x80x9cVxe2x80x9d thread interface created by these xe2x80x9cVxe2x80x9d threads converts the rotation of the stem into translation of the gate. In addition, the xe2x80x9cVxe2x80x9d thread interface between the stem and the gate is preferably sufficiently loose to allow the gate to float laterally to engage the downstream seat when the gate valve is in the closed position.
In accordance with yet another embodiment of the present invention, the gate valve comprises a bearing support ring which is rotationally received within the bonnet cap and includes an axial bore through which the first portion of the valve stem is inserted, a bearing pin for securing the bearing support ring to the stem, and a preferably non-metallic thrust bearing washer which is disposed between the bearing support ring and the bonnet cap. The bearing support ring, bearing pin and thrust bearing washer comprise an inexpensive, lightweight assembly that effectively transmits the thrust from the valve stem to the bonnet. In addition, the non-metallic thrust bearing washer will not rust and does not require the application of lubrication during the life of the gate valve.
In accordance with still another embodiment of the invention, the gate cavity comprises a first chamber located below the flow passage and a second chamber located above the flow passage, and the cross sectional area of the first chamber in a plane perpendicular to the valve stem is less than the cross sectional area of the second chamber in a plane perpendicular to the valve stem. Thus, the size of the valve body below the flow passage is minimized to reduce the overall weight and cost of the gate valve, as well as reduce the volume of trapped pressure in the gate valve that contributes to problems with blow down when the gate is opened. In addition, the wall sections of the valve body preferably comprise a generally uniform thickness to improve casting flow during manufacture of the gate valve.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings.